Cross-Reference to Related Application
Reference is hereby made to the copending application of Roger L. Frick, Ser. No. 899,378 dated Aug. 22, 1987, now issued as U.S. Pat. No. 4,782,659, assigned to the same assignee as this application.
1. Field of the Invention
The present invention relates to transmitters used in industrial process control systems.
2. Description of the Prior Art
Two-wire transmitters (as well as three-wire
widespread use in and four-wire transmitters) find industrial process control systems. A two-wire transmitter includes a pair of terminals which are connected in a current loop together with a power source and a load. The two-wire transmitter is powered by the loop current flowing through the current loop, and varies the magnitude of the loop current as a function of a parameter or condition which is sensed. Three and four wire transmitters have separate leads for supply current and outputs. In general, the transmitters comprise energized electrical circuits which are enclosed in a sealed housing such that ignition of any combustible atmosphere by faults or sparks from the energized circuit is contained in the housing.
Although a variety of operating ranges are possible, the most widely used two-wire transmitter output varies from 4 to 20 milliamperes as a function of the sensed parameter. It is typical with a two-wire transmitter to provide adjustment of the transmitter so that a minimum or zero value of the parameter sensed corresponds to the minimum output (for example a loop current of 4 milliamperes) and that the maximum parameter value to be sensed corresponds to the maximum output (for example 20 milliamperes).
The minimum and maximum parameter values will vary from one industrial process installation to another. It is desirable, therefore, to provide some means for setting the maximum and minimum output levels in the field, and this is done typically with electrically energized zero and span potentiometers sealed in the housing. With some transmitters, a housing cover must be removed to gain access to the potentiometers for adjustment, undesirably exposing the atmosphere surrounding the transmitter to the live circuits in the transmitter. A variety of techniques, however, are available for adjusting the potentiometers while sealing potentially explosive atmospheres surrounding the transmitter from the electrically live circuits in the transmitter. In some transmitters, a rotary adjustment shaft for adjusting a potentiometer is closely fitted through a bore in the housing to provide a long flame path for quenching ignition in the housing before it reaches the atmosphere surrounding the housing. In yet another arrangement, the potentiometers are mechanically coupled to a relatively large bar magnet which is then rotated magnetically by another bar magnet outside the live circuit's enclosure. This arrangement with bar magnets can have the disadvantage of mechanical hysteresis, making precise span and zero setting difficult. Actuated switches are also used for setting span and zero in transmitters, such switches requiring an opening through the wall of the transmitter's housing to provide for mechanical coupling to the switch.
For many process control environments, the transmitter itself is required to have an explosion-proof enclosure. This means that, if a spark takes place inside of the transmitter housing which ignites gases within the housing, no hot gases should be propagated from the interior of the transmitter to the exterior which could cause any surrounding combustible atmosphere to ignite.
Providing for zero and span adjustments which are accessible from outside the transmitter (so that the housing would not have to be opened) is desirable, but makes it difficult to maintain the explosion-proof characteristics of the transmitter. External span and zero actuators have, in the past, needed either bulky magnet pairs for transmitting rotational force or passages formed through the transmitter housing wall, so that one end of the actuating mechanism extends into the chamber which contains the transmitter electronics, while the other end is accessible from the exterior of the transmitter. In order to maintain explosion-proof characteristics, very long flame paths must be created with very tight tolerances. It is also important that the passages be sealed so that moisture cannot enter the transmitter housing through the span and zero actuator passages.
There is a continuing need for improved zero and span actuators which are easier to fabricate, require less critical tolerances, and are less expensive than prior art actuators.